1. Field of the Invention
The present invention relates to an ND filter for an aperture device which is suitable for use in a photographing optical system such as a video camera or still video camera, and an aperture device comprising the ND filter.
2. Description of the Related Art
An aperture device mounted in a conventional video camera serves to control the quantity of light entering a solid-state image pickup element, and is stopped down to have a small aperture size when the object field is a high-luminance object field. Thus, on a very fine day or when photographing a high-luminance object, the aperture size becomes small. Because of diffraction of light due to the small aperture, degradation of the image quality may occur.
An example solution for this problem is attaching a film-like ND (Neutral Density) filter to an aperture blade. Then, even if the object field is bright, the aperture does not become smaller than a predetermined size. Namely, when the aperture becomes small, the ND filter is positioned in the optical path to reduce the quantity of light. This prevents the aperture from becoming extremely small even when photographing a high-luminance object.
For example, Japanese Patent No. 2592949 discloses an ND filter in which the transmittance increases stepwise toward the optical axis. Japanese Patent Application Laid-Open No. 2004-117467 discloses an ND filter in which the transmittance increases continuously toward the optical axis. When moving an ND filter having a stepwise density change or density gradation or the like on the optical axis, one can adjust the light quantity without changing the aperture size.
Such an ND filter employs a film-like plastic substrate made of a plastic material, e.g., PET or PEN, which is a material having good optical characteristics and high durability. A multi-layered thin film, obtained by alternately stacking thin film made of a light-absorbing material and thin film for the reduction of reflectance, is formed on the surface of the plastic substrate by the evaporation method or the like.
In the evaporation process of manufacturing the ND filter, however, the longer the time of evaporation process of depositing evaporation particles, melted and evaporated from an evaporation source, on the surface of the plastic substrate becomes, the higher the temperature of the plastic substrate will rise. Thermal expansion coefficient of the plastic substrate and that of the thin film stacked on the plastic substrate are not always equal. The multi-layered thin film may thus interfere with thermal expansion and contraction of the plastic substrate. As a result, deformation such as wrinkles occurs in the finished ND filter. Such a product is not appropriate as an ND filter.
As a method of suppressing deformation such as wrinkles in the ND filter, for example, evaporation process may be performed with a pattern producing mask to form a thin film into a desired shape, being in tight contact with the plastic substrate. This can decrease deformation of the plastic substrate caused by thermal expansion and contraction.
The techniques disclosed in Japanese Patent Application Laid-Open No. 2004-37545, 2004-37548, and 2005-62903 employ a plastic substrate which is made of a norbornene-based plastic material having a glass transition temperature of 120° C. or more. The temperature of the substrate in the evaporation process is maintained lower than the glass transition temperature of the norbornene-based plastic material to suppress deformation caused by thermal expansion and contraction, thus preventing winkles.
Recently, an improvement in sensitivity of the image pickup element leads to a demand for a high-density ND filter having a much smaller light transmittance. To obtain such optical characteristics, the arrangement of the multi-layered film must be changed. More specifically, such schemes as increasing the thickness of a predetermined layer of the multi-layered film, or increasing the number of layers in the multi-layered film is employed. Accordingly, during the evaporation process the temperature of the plastic substrate rises higher, and the high-temperature period in the vacuum chamber becomes longer. In this case, even when evaporation is performed with the pattern producing mask, being in tight contact with the plastic substrate, it cannot suppress deformation of the plastic substrate caused by thermal expansion and contraction.
Each ND filter as disclosed in Japanese Patent Application Laid-Open Nos. 2004-37545, 2004-37548, and 2005-62903 has the substrate made of the norbornene-based plastic material. In the evaporation process of forming a high-density ND filter having a density of, e.g., 1.5 or more, the substrate temperature largely exceeds 120° C., and sometimes reaches almost 200° C. An ordinary norbornene-based plastic material has a glass transition temperature of about 120° C. to 170° C. In the evaporation process to manufacture such a high-density ND filter, the substrate temperature becomes higher than this glass transition temperature. If an ND filter employing a norbornene-based plastic substrate is manufactured under these conditions, a number of wrinkles appear on the portion of the ND filter on which the multi-layered thin film is formed. It is very difficult to eliminate these wrinkles.